Surface alloys with various compositions of Co, Cr and W on 20CrMo steel have been processed by controlling the ratio of thickness of plasma sprayed Co-Cr-W coating to laser melting depth. A formula has been suggested to evaluate the contents of surface alloying elements. With increasing the contents of Co, Cr and W in the surface alloy system, two hardness peaks occur at about 17Co7Cr3W and 52Co22CrgW corresponding to microstructure of M and γ-Co + M23C6 respectively. Wear experiments in media of air, 1N H2SO4 and 1% NaCl indicate that 17Co7Cr3W surface alloy has higher wear resistance in air, while 52Co22Cr9W better in the corrosive media under high load. In any case, wear resistance of the above two surface alloys are much better than that of plasma sprayed coating of Co-Cr-W.